Waterjet Cutting – Deepening a hole and cautions with glass

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

The last three posts have described what happens when a jet of water first arrives on a surface and then starts to penetrate into the material. At a close stand-off distance, the erosion starts around the edge of the jet and continues to widen the hole as it gets deeper, until a point where the pressure at the bottom of the hole falls and the jet stops going deeper. The lateral flow away from the bottom of the jet continues to erode material, however, and so the hole gets a little wider at the bottom. This creates a small chamber under the entrance hole and this can build up enough pressure that it can cause the material around the hole to break.

Progress in the high-pressure waterjet drilling of a hole in rock

Figure 1. Progress in the high-pressure waterjet drilling of a hole in rock.

In the last post I showed where this happened with a 1-ft cube of rock that had been broken with a single pulse, but this fracture of the target can occur when piercing glass or other brittle materials. So the question becomes how to stop the fracture if one is trying to cut glass. This applies when the job calls for making an internal cut in the glass, and not when cutting in from the side, although that also has some problems that I will address in a later post.

When starting an internal cut, it obviously means piercing a starter hole through the glass in a region that is going to be part of the scrap, if this is possible, as it would be, for example, when cutting a sculpture. A secondary reason for that location, apart from confining any small cracks that might happen during the pierce, is that these starter holes are larger in diameter (for the reason given above) than the cut line once the jet starts to move, and that hole section would appear as a flaw on a final cut line.

Vanessa Cutler, in New Technologies in Glass discusses the process of cutting in more detail but suggests that the starter hole be pierced at a lower pressure than that to be used in the cut. This is so that the pressure within the cavity will remain lower during the pierce, and insufficient to cause the glass to break. She suggests (and she has a vastly greater experience than I in this) that the piercing pressure be around 11,000 to 18,000 psi – this varies a bit with abrasive grit size, machine size and glass type.

Detail of the glass sculpture “p1″, by Vanessa Cutler

Figure 2. Detail of the glass sculpture “p1″, by Vanessa Cutler. (Note that these holes do not pierce all the way through the glass but all end at the same depth.)

She also recommends, when there are multiple cuts to be made on a sheet, that all the piercing holes be completed before any cutting begins. One of the reasons for this is to avoid constantly resetting the cutting pressure, which could be a problem if you forget to lower the pressure back down before starting the next pierce. (Would I as an Emeritus Professor ever be that absent-minded? Why else bring it up?)

You will notice, with abrasive cutting into glass, that there is not the belling at the bottom of the cut like with plain waterjet cutting and that the hole tapers with depth as the cutting effectiveness reduces with the fall in pressure with depth; and the jet is less able to cut into the side walls of the opening at these lower pressures.

Stepping back from the cutting of glass to the more general condition where the jet runs out of power at the bottom of the hole, the main reason for this is the conflict between the water in the fresh jet coming into the hole and the spent water trying to make it out of the hole at the same time.

One way of overcoming the problem is to interrupt the flow of water into the hole. Back in my grad student days, we tried doing this by breaking the jet into slugs, so that one slug would have enough time to travel to the bottom of the hole, cut a little, and then rebound out of the hole, before the next slug of water arrived. There was relatively little sophistication in the tool we designed to do this. Simply it was a disk, with holes drilled in it at an angle.

Interrupter disk placed in the path of a continuous jet

Figure 3. Interrupter disk placed in the path of a continuous jet. (My PhD Dissertation)

The reason for the angled holes was to make the disk self-propelling as it rotated under the jet, since the angled edges of the hole forced the disk to continue rotating once started. (On a minor note, the disk would rotate at several thousand rpm, and the noise that it made was loud enough that I was instructed to only carry out the tests after the staff had left for the evening).

The penetration of a waterjet into sandstone with the jet running continuously (black), with the jet interrupted (red) and with the jet rotated slightly off-axis (green)

Figure 4. The penetration of a waterjet into sandstone with the jet running continuously (black), with the jet interrupted (red) and with the jet rotated slightly off-axis (green). (Brook, N. and Summers, D.A., “The Penetration of Rock by High Speed Waterjets”, Intl. Journal Rock Mechanics and Mining Science, May, 1969)

As can be seen in figure 4, with the pulsating jet more energy was getting to the bottom of the hole without interference, and the hole continued to deepen over time. However, the interruption tool had a number of disadvantages, apart from the noise and that the disk would be very rapidly destroyed under an abrasive jet. It was wasting a significant portion of the energy: In a more optimized design that I won’t discuss further, the energy loss was about 50%.

So it would be best if the jet was moved slightly over the surface, and in these early tests, the easy way to do this was to have the target rotate with the jet hitting the rock just offset from the axis of rotation. (At the time high-pressure swivels weren’t yet available). This gave the upper curve in figure 4, and a much more rapid penetration of the target.

In more modern times, the nozzle is moved either by causing it to move slightly around the hole axis or by causing a slight oscillation or “dither” in the nozzle while the pierce is taking place. This is generally a feature of the control software that drives the cutting table. But the reason for the movement is to get the water flowing in such a way that the water going out of the hole does not interfere with that going in, and so there is a reduced risk of pressure build-up in the hole, with the consequent cracking that this would cause.

Waterjetting – The Effect of Standoff Distance

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

One of the problems with relying on photographs is that they are sometimes not of the quality that one would wish. This has happened with today’s topic, where the pictures are old, smaller and in poorer condition than I had remembered. However, with your indulgence, I am going to step through them. I do apologize for their poor quality, however.

The topic is the way in which a waterjet first attacks a target. I have mentioned different parts of this process in the past. But in this post I want to show that it matters where the target is, relative to the nozzle, because the structure of the jet itself changes with that distance, which I call the standoff distance between the jet orifice and the initial target surface.

The break-up pattern of a waterjet

Figure 1. The break-up pattern of a waterjet (Yanaida K. “Flow Characteristics of Waterjets,” 2nd BHRA Conf. 1974, paper A2.)

As I mentioned last time, when the target is close to the nozzle, then the erosion pattern can, in the first few seconds of contact, be seen to be like a butterfly in pattern. The central part of the target under the jet is not eroded but there is severe erosion around the edges of the jet diameter, where a grain will see high differential pressures across its width and will be subject to high lateral jet flows.

Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle

Figure 2. Damage pattern around the impact point of a 10,000 psi pressure, 0.04 inch diameter jet on aluminum, target close to the nozzle

As the nozzle is moved away from the target surface, however, that pattern of erosion changes. As the jet structure picture shows, the central zone at the initial pressure reduces in radius, and there is an intermediate zone of rapidly diminishing pressure, with an outer shroud of fine droplets. The effect on the impacted target is that there continues to be a small zone with no erosion in the center, and that erosion is still concentrated around this zone, in that of high differential pressure, which now encroaches on that central sector.

Erosion of an aluminum target with the nozzle 2-inches above the surface

Figure 3. Erosion of an aluminum target with the nozzle 2-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

That central small plateau is reduced to a very small point by the 3-inch standoff, which is where the jet reaches the end of the distance where the pressure remains constant over the central section. Thus, by a 4-inch standoff, the central section, though still present, is being eroded.

Erosion of an aluminum target with the nozzle 4-inches above the surface

Figure 4. Erosion of an aluminum target with the nozzle 4-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

As the nozzle is moved further back from the surface, that central promontory disappears at around a six-inch standoff. It is interesting to note that at this point, the cavity is starting to get noticeably deeper.

Erosion of an aluminum target with the nozzle 6-inches above the surface

Figure 5. Erosion of an aluminum target with the nozzle 6-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice. (The lower of the two circular damage patterns was caused through experimental conditions and should be ignored). The presence of a central mound can barely be discerned.

By this time, the central section of the jet is beginning to break down into initially short strings that very rapidly break into droplets. The damage pattern that results shows a cavity that is slightly increasing both in diameter and depth.

Erosion of an aluminum target with the nozzle 8-inches above the surface

Figure 6. Erosion of an aluminum target with the nozzle 8-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

By this time, the jet is continuing as a series of relatively large droplets, still holding a central structure, though surrounded by a rapidly decelerating cloud of mist.

Erosion of an aluminum target with the nozzle 10-inches above the surface

Figure 7. Erosion of an aluminum target with the nozzle 10-inches above the surface, 10,000 psi jet through a 0.04 inch diameter orifice

It is one of the interesting oddities of the jet cutting business that the amount of material that is eroded from the target is a maximum at this distance.

However, and this was the subject of great debate back at the time that it was first presented, the ability to control the droplet size and condition as a function of distance and the reality that in most applications the target must be cut to depth meant that this has a very limited application. It can be used if the droplets are generated properly and used within the relatively narrow window that they exist to improve surface erosion of material.

However, as Mike Rochester found when he studied this, back at Cambridge in the early 1970’s, the presence of a layer of water on the surface, and as the hole deepens this is almost always there, rapidly diminishes the effect.

The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface

Figure 8. The effect of a layer of water in diminishing the “droplet impact” effect in erosion of a surface. (After M.C. Rochester, J.H.Brunton “High Speed Impact of Liquid Jets on Solids” First BHRA Symp Jet Cutting Tech, April `972, Coventry UK, paper A1.)

There are ways of getting around this problem but the presence of water in the cavity that the jet has produced can also lead to problems, and these will be the topic of the next two posts.

Waterjet Technology – Water Jet Stream Structure

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

In last week’s post I showed some high-speed photographs of the plain water jets that come from the small diamond and sapphire orifices and that are useful in cutting a wide variety of target materials. Before moving away from the subject of high-speed photography, this post will use results from that technique to talk about why pressure washer nozzles may not work well and have limited range. From there it will raise the topic of adding abrasive to a waterjet stream.

Most of us, I suspect, by this point in time, have used a pressure washer to do some cleaning, typically around the house or perhaps at a car wash. The jet that comes out of the end of the nozzle is typically a fan-shaped stream that widens as the water moves away from the orifice. This flattening of the jet stream and the resulting spreading jet is achieved by cutting a groove across the end of the nozzle to intersect either a conic or ball-ended feed channel from the back end of the nozzle.

Schematic of how a fan–generating orifice is often made

Figure 1. Schematic of how a fan–generating orifice is often made

One of the problems with this simple manufacturing process is that the very sharp edge that is produced to give a clean jet leaving the nozzle is very thin at the end. This means that with water that is not that clean (and most folk don’t filter or treat pressure washer water) the edge can wear rapidly. I have noted several designs (and we tested many) where the jet lost its performance within an hour of being installed, particularly with softer metal orifices. And in an earlier post, I did show the big difference between the performance of a good fan jet and a bad.

So how do photographs help understand the difference and explain why you should generally keep a fan jet nozzle within about 4-inches of a surface if you are trying to clean it? That does, however, depend on the cone angle that the jet diverges at once it leaves the nozzle. We found that a 15-degree angle seemed to work best of the different combinations that we tried. If the jet remained of sufficient power, this would mean that it would clean a swath about half-an inch wide with the nozzle held 2-inches above the surface. At 4-inch standoff it will clean a swath about an inch wide and at 6 inches this goes up to over an inch-and-a-half. But that would require that the jet be of good quality and evenly distributed.

Back-lit flash photograph of a fan jet

Figure 2. Back-lit flash photograph of a fan jet, at a jet pressure of around 1,000 psi. It is less than 6 inches from the end of the orifice to the rhs of the picture.

In Figure 2, the lack of water on the outer edges of the stream shows that the water is not being evenly distributed over the fan. As the water volume leaves the orifice, the sheet of water begins to spread out into the wider but thinner sheet that forms the fan. But as it gets wider it also gets thinner, and, like a balloon, water can only be spread so thin before the sheet begins to break up. As soon as it starts to do so, the surface tension in the water causes it to pull back into roughly circular rings of droplets.

Fan jet breakup from a spreading sheet into rings (or strings) of large droplets that rapidly break down into mist

Figure 3. Fan jet breakup from a spreading sheet into rings (or strings) of large droplets that rapidly break down into mist.

These droplets start out as relatively large in size, but they are moving at several hundred feet per second. As single droplets move through stationary air, the air rapidly breaks them up into smaller droplet sizes and then into mist while at the same time slowing the droplets down. The smaller they get, the quicker that deceleration occurs. When droplets get below 50 microns in size, they become ineffective. (From a study that was done on determining the effect of rain on supersonic aircraft).

Showing the stages of the fan jet breakup from a solid sheet to mist that does little but wet the surface that it strikes

Figure 4. Showing the stages of the fan jet breakup from a solid sheet to mist that does little but wet the surface that it strikes.

However, if the nozzle is held just in that short range where the droplets have formed but have not broken down, then the jet will be more effective than it would have been at any other point along its length. This is because of something that was first discovered when scientists at the Royal Aircraft Establishment-Farnborough and at the Cavendish Lab at Cambridge University were studying what would happen if they flew a Concorde into rain while it was still going supersonic. (They actually tried this in a heavy rain storm in Asia and found it was a seriously bad idea).

The pressures that can develop under the spherical droplet can exceed twice the water hammer pressure so that the impact pressure on the surface can exceed 20-times the driving pressure supplied by the pump. But the region affected is very small, and the effect diminishes as the surface gets wetter. And the problem, as with all waterjet streams, is that it is very hard to know where that critical half-inch range is. It varies even within the same nozzle design models due to small changes on the edge of the orifice. And as a very rough rule of thumb, a perfect droplet moving at a speed of around 1,000 ft/sec will travel 138 diameters before it is all mist. Most drops aren’t perfect and thus will travel around 30 – 50 diameters and once they turn into mist they will decelerate to having no power in less than quarter-of-an-inch. The implication of this, which we checked with field experiments, is that if you hold a pressure washer nozzle with a fan tip more than 4-6 inches from the target you are largely just wetting the surface, and spending a fair amount of money in creating turbulent air.

This story of jet breakup is a somewhat necessary introduction to two posts that I will publish before long. The first will be to discuss how we can use a different idea for nozzle designs to do a much better job at greater standoff distances and I will tie that in with some of the advantages of going to much higher pressure to do the cleaning job.

The other avenue that this discussion opens relates to how we mix abrasive within the mixing chamber of an abrasive nozzle design, and that will come along a little later.

(For those interested in more reading, there has been a series of Conferences on Rain Erosion, and then “Erosion by Solid and Liquid Impact” which were held under the aegis of John Field at Cambridge for many years, e.g. Field, J.E., Lesser, M.B. and Davies, P.N.H., “Theoretical and Experimental Studies of Two-Dimensional Liquid Impact,” paper 2, 5th International Conference on Erosion by Liquid and Solid Impact, Cambridge, UK, September, 1979, pp. 2-1 to 2-8. The founding conference was held under the imprimatur of the Royal Society, which devoted a volume to the Proceedings. Phil. Trans. Royal Society, London, Vol. 260A.)

Waterjetting Technology – Pipe Straighteners

By Dr. David A. Summers, Curators’ Professor at Missouri University of Science & Technology

KMT Waterjet Systems Weekly Waterjet Blog

KMT Waterjet Systems Weekly Waterjet Blog

One of the advantages that became clear, even in the early days of waterjet use in mining, was that the jets cut into the rock away from the miner. It was thus a safer method of working, since it moved the person away from the zone of immediate risk. Rock has a tendency to fall when the rock under it is removed, and by using the jets to carry out the removal, so the miner is no longer as vulnerable.

But in the early days of jet use, the range of the jet was quite limited. Part of the reason for this is that the water is generally brought to the working place along the floor. It then has to be raised through bent pipes to the level of the nozzle and then turned so that the water in the pipe is flowing in the direction in which the nozzle is pointing.

Sketch of an early Russian waterjet mining monitor

Figure 1. Sketch of an early Russian waterjet mining monitor

Even though the pressure of the jet is relatively low, the volume flow rates were high and the bends leading into the nozzle set up considerable turbulence in the jet, so that the range of the jet was quite limited beyond the nozzle. There are a number of different ways of improving the range of the jet, and I will discuss these in later posts; many of these techniques apply whether the jet is being used at high volume and low pressure for mining or at higher pressures and lower flow rates for cutting into materials. But today the technique that I will discuss is the use of flow straighteners.

The two most dramatic instances that I immediately recall for their use were at the Sparwood mine in British Columbia, where the collimated jet was able to mine coal up to more than 100 ft. from the nozzle and in an underground borehole mining application where a Bureau of Mines commissioned system was able to cut a cavity to more than 30 ft. from the nozzle, which was centrally located.

Collimating jets to get better performance is not restricted to the mining industry. A visit to Disney, for example, will find jumping jets that appear to bounce from place to place (video here) (this one shows the start of the surface waves along the jet, known as Taylor instability, which grow and cause the jet to break up; and if you want to make one Zachary Carpenter has two instructional videos on how they are made (here and here).

Essentially, as those YouTube segments show, the flow straightness is achieved by dispersing the water – using a sponge – so that it flows through a large number of drinking straws. These straws act to collimate the water flow and it emerges as a glassy rod, which even acts as a light path so that light shone down it emerges at the far end. This can be used for a variety of different purposes, other than just for entertainment.

This then is the basic idea behind a flow collimator, although for larger mining flows drinking straws are too weak, and the flow volumes need to be larger. There are various designs that have been used for mining applications. Some of the earlier trials were at the Trelewis Drift mine, where the then British National Coal Board set up an experimental operation.

Sketch of Monitor used in the NCB Trials

Figure 2. Sketch of Monitor used in the NCB Trials (after Jenkins, R.W., “Hydraulic Mining” The National Coal Board Experimental Installation at Trelewis Drift Mine in the No 3 Area of the South Western Division, M.Sc. Thesis, University of Wales, 1961.)

A number of different designs were used for the flow straighteners that were located at the nozzle end of the straight pipe section leading into the nozzle:

Designs for the initial flow straighteners used at Trelewis Drift

Figure 3. Designs for the initial flow straighteners used at Trelewis Drift (after Jenkins, R.W., “Hydraulic Mining” The National Coal Board Experimental Installation at Trelewis Drift Mine in the No 3 Area of the South Western Division, M.Sc. Thesis, University of Wales, 1961.)

More recent designs, which vary according to pressure, flow rate and pipe diameter are a combination of those on the left above and those on the right. It was such a combination that allowed the Canadian miners at Sparwood to achieve production rates of 3,000 tons of coal per shift as an average over the operation of a mining section.

While the use of flow straighteners does not give any gain over having a long straight section of pipe leading into the nozzle, it can bring the flow condition up to that level in places where the geometry (or the resulting unwieldiness of the pipe) would make the long entry impractical.

One of the more interesting applications of this is in the borehole mining of minerals. Simplistically, a hole is drilled from the surface down to the seam of valuable mineral. Then a specially designed pipe is lowered through the hole with the pipe having a nozzle set on the side. Then, as the pipe rotates and is raised and lowered, the jet mines out the valuable mineral, which flows to the cavity under the pipe, where it is sucked into a jet pump and carried to the surface.

Schematic of a borehole mining operation

Figure 4. Schematic of a borehole mining operation (George Savanick)

As I mentioned at the top of the article, the jet cut a cavity some 30 ft in radius with the jet issuing through a nozzle some 0.5 inches in diameter. In order to achieve this range, it was important that the jet was properly collimated, yet the nozzle was set so that there could be no straight section.

Section showing the feed into the borehole miner nozzle

Figure 5. Section showing the feed into the borehole miner nozzle. Note the vanes in the section leading into the nozzle (George Savanick).

The turning vanes to achieve the flow collimation were designed by Lohn and Brent (4th Jet Cutting Symposium) to produce a jet equivalent to that achieved had the nozzle been attached to a straight feed.

Turning vanes used to achieve a jet capable of cutting coal to 30-ft from the nozzle

Figure 6. Turning vanes used to achieve a jet capable of cutting coal to 30-ft from the nozzle. (P.D. Lohn and D.A. Brent “Design and Test of an Inlet Nozzle Device” paper D1, 4th Int Symp on Jet Cutting Technology, Canterbury, BHRA 1978)

Tests of the performance of the nozzle showed that it produced a jet that was at least equal in performance to a nozzle with a straight feed, up to a standoff distance of 45 ft.

In simpler applications, the designs do not need to be that complicated for many simple spraying nozzles, for example, the straightener is made up of a simple piece of folded metal.

Simple flow straightener for use in low pressure and flow applications

Figure 7. Simple flow straightener for use in low pressure and flow applications.

The water has now reached the nozzle, but that is not the end of the story of the feed system, as I will start to explain, next time.